1. Field of the Invention
The present invention relates to a method for forming a thin film insulated gate type field effect transistor (thin film transistor, TFT). The TFT formed by the present invention is used as a switching element of a pixel in a liquid crystal display device, or in a semiconductor integrated circuit.
2. Description of the Related Art
In a thin film device such as a thin film insulated gate type field effect transistor, when a gate electrode does not overlap with source and drain regions to obtain an offset state, it is well known that a leakage current between the source and drain regions can be reduced.
In particular, it is desired that an offset region which the gate electrode does not overlap with the source region or the drain region has a width of about sub-micron order and high precision. If the width of the offset region is changed by 0.5 .mu.m or more, characteristic of a thin film transistor (TFT) has changed entirely. Accordingly, it is desired that the width of the offset region is 0.1 .mu.m or less, preferably 500 .ANG. or less. However, such precision cannot be obtained in photolithography.
To solve this point, the following method is proposed. That is, a gate electrode is formed by using an anodizable material and anodized, so that anodic oxide having a thickness of 5 .mu.m or less is formed in at least sides of the gate electrode by precision of 100 .ANG. or less. Using the anodic oxide as a mask, an impurity is introduced by an ion doping, an ion implantation, or the line, to obtain an offset state in self-alignment.
If a gate electrode is formed by a metal material containing mainly aluminum, it is easy to anodize and useful to improve circuit characteristics because of a low resistance. However, if aluminum is anodized, anodic oxide is not stoichiometric oxide (Al.sub.2 O.sub.3) and has a low oxygen concentration, in general. This means that metallic aluminum contained in the anodic oxide presents isolatedly. Even though such aluminum is matter such as atom, matter such a cluster which a plurality of atoms are collected, or further large lump, electron is easily emitted and it tends toward ionization. Therefore, the anodic oxide tends to attract the electron macroscopically. That is, if the anodic oxide presents on a gate insulating film, since negative charge is inducted in a semiconductor active layer of an offset region formed under the gate insulating film, the offset region becomes an N-type, so that an offset region having an I-type cannot be formed.
From the above, there are a defect that a leakage current between source and drain regions gets large in an N-channel type TFT and an absolute value of a threshold voltage gets high in a P-channel type TFT. Also, since degree of electrolytic dissociation with respect to the metallic aluminum contained in the anodic oxide is changed dependent on an use time of a TFT, there is a defect with respect to reliability that characteristic is not stable.